1. Field of the Invention
The present invention relates to a tunable radio-frequency unit for a magnetic resonance device with a radio-frequency antenna and a radio-frequency signal line connected thereto.
2. Description of the Prior Art
U.S. Pat. No. 4,801,885 teaches a tunable radio-frequency unit of the above-described type. The radio-frequency unit belongs to a diagnostic magnetic resonance device and is constructed for exciting and receiving magnetic resonance signals in a patient. The radio-frequency unit includes a radio-frequency antenna which is formed by two subsystems. The two subsystems are connected to a common terminal point of a matched connection via respective radio-frequency signal lines. At the common terminal point, a .lambda./4 stub is additionally connected which can be operated selectively at its ends with a switch to represent a short circuit or a no-load condition. The designation .lambda. stands for the wavelength of a radio-frequency signal, given the operating frequency on the stub. The .lambda./4 stub transforms a short circuit at the end of the stub into a no-load at the common terminal point, as well as transforming a no-load at the end into a short circuit at the common terminal point. The short circuit effects a tuning of the connected radio-frequency antenna. This is necessary if another antenna is used for transmitting and receiving a magnetic resonance signal. A good decoupling of the two antennas is achieved by means of the tuning. It is a disadvantage, however, that the .lambda./4 transformation line requires additional structural volume.
Another possibility for realizing a tunable radio-frequency unit is to short-circuit resonance capacitors (shorting capacitors) belonging to the radio-frequency antenna so as to tune the antenna. To avoid disturbing interactions with the magnetic resonance device, pneumatically operated switches, for example, are utilized for this purpose. Such switches, however, require a pneumatic feed line to the magnetic resonance device. There are also tuning circuits using PIN diodes, however, they must be operated for this purpose with a very high bias voltage on the order of magnitude of 500 V. Besides, the tuning effect of the PIN diode circuit is limited, due to the short circuit, which is not ideal.